Numerical Modelling of Multi-directional Earthquake Loading and Its Effect on Sand Liquefaction

Earthquakes generate multi-directional ground motions, two components in the horizontal direction and one in the vertical. Nevertheless, the effect of vertical motion on site response analysis has not been the object of extensive research. The 2010/2011 Canterbury sequence of seismic events in New Zealand is a prime example among other earlier field observations strongly corroborating that the vertical acceleration may have a detrimental effect on soil liquefaction. Consequently, this study aims to provide insight into the influence of the input vertical motion on sand liquefaction. For this reason, two ground motions, with very different frequency contents, are used as the input excitations. Non-linear elasto-plastic plane strain fully coupled effective stress-based finite element analyses are conducted to investigate the occurrence of liquefaction in a hypothetical fully saturated Fraser River Sand deposit. The results indicate that the frequency content of the input motion is of utmost importance for the response of sands to liquefaction when the vertical loading is considered

The importance of accurate time-integration in the numerical modelling of P-wave propagation

The numerical dissipation characteristics of the Newmark and generalised-α time-integration schemes are investigated for P-wave propagation in a fully saturated level-ground sand deposit, where higher frequencies than those for S-waves are of concern. The study focuses on resonance, which has been shown to be of utmost importance for triggering liquefaction due to P-waves alone. The generalised-α scheme performs well, provided that the time-step has been carefully selected. Conversely, the dissipative Newmark method can excessively damp the response, changing radically the computed results. This implies that a computationally prohibiting small time-step would be required for Newmark to provide an accurate solution

On the use of nonlocal regularisation in slope stability problems

This study examines the use of nonlocal regularisation in a coupled consolidation problem of an excavated slope in a strain softening material. The nonlocal model reduces significantly the mesh dependency of cut slope analyses for a range of mesh layouts and element sizes in comparison to the conventional local approach. The nonlocal analyses are not entirely mesh independent, but the predicted response is much more consistent compared to the one predicted by local analyses. Additional Factor of Safety analyses show that for drained conditions the nonlocal regularisation eliminates the mesh dependence shown by the conventional local model

A case study on the seismic performance of earth dams

The seismic non-linear behaviour of earth dams is investigated by using a well-documented case study and employing advanced static and dynamic coupled-consolidation finite-element analysis. The static part of the analysis considers the layered construction, reservoir impoundment and consolidation, whereas the dynamic part considers the response of the dam to two earthquakes of different magnitude, duration and frequency content. The results of the analysis are compared with the recorded response of the dam and exhibit a generally good agreement. The effects of the narrow canyon geometry, the reservoir impoundment and the elasto-plastic soil behaviour on the seismic dam behaviour are investigated. Finally the implications of the adopted constitutive modelling assumptions on the predicted response are discussed